Electron sensing devices, or electron bombarded devices rely on high energy electrons to generate gain by a cascade or knock-on process. One consequence of these high energy electrons is the probability that they may be backscattered upon impact with the electron collection surface of the device. The backscattered electrons produce a loss in signal and spatial resolution.
There is a class of devices that use high energy electrons bombarding a surface to produce gain and amplify a small signal. Examples of such devices are hybrid photodiodes (HPDs), electron bombarded active pixel sensors (EBAPSs), electron bombarded CCDs (EBCCDs), electron bombarded metal-semiconductor-metal (MSM) vacuum phototubes (MSMVPTs), avalanche photo diodes (APDs) and resistive anodes. For the cases of EBAPS and EBCCD, spatial resolution is paramount to maintain image quality. Signal strength is also a factor for low light level imaging. Although spatial resolution is less important for HPDs and MSMVPTs, signal integrity is an overriding factor, as the devices require single photon detection and high speed. Even so, spatial resolution is important for segmented photodiodes.
A consequence of using high energy electrons is that a fraction of the primary electrons are backscattered. If the backscattered electron does not land on the detector, then signal is lost, but there is no spatial degradation. If the backscattered electron, however, lands again on the detector, then the signal level is maintained, but it is spatially displaced from the original impact point.
Typically, these bombarded devices have planar semiconductor surfaces, and the high energy electrons impact these planar surfaces. A portion of the high energy electrons are backscattered. The backscattered electrons may be considered as being reflected, much like light is reflected from a surface of a solar cell. In a solar cell, anti-reflection coatings (ARCs) are used to reduce the reflection of the light. Electron bombarded device, however, cannot use ARCs, because ARCs attenuate the power of the incident signal and, therefore, reduce gain of the devices. An alternative to ARCs in solar cell technology is use of textured surfaces. Textured surfaces are used to decrease reflection from surfaces of highly efficient solar cells.
There are three objectives in designing solar cells: (1) reduce the front reflection, (2) increase the path length, and (3) trap weakly absorbed light reflected from the back. In the case of electron bombarded surfaces, however, the last objective is not applicable, due to the very short path length of the high energy electrons. Although textured surfaces have successfully been used in the field of solar cells to improve light absorption, textured surfaces have not been used in the field of electron bombarded devices to reduce backscattering of electrons and reduce halos in the output images.
In U.S. Pat. No. 6,005,239, issued on Dec. 21, 1999, Suzuki et al. disclose an image intensifier including a transparent entrance faceplate, and an optical fiber block. The fiber block is made of many optical fibers bundled together, and is disposed opposite to the entrance faceplate. A vacuum atmosphere is formed between the entrance faceplate and the optical fiber block. The optical fiber block is provided with pits, in which each pit includes an end face of a core portion of an optical fiber that is recessed from an end face of a cladding portion of the optical fiber. The cladding portion projects from the surface of the recessed core portion, thereby forming a pit. Accordingly, Suzuki et al. teach formation of pits in an optical fiber block, which are made of many optical fibers bundled together for reducing the halo phenomenon of output light.
A need exists for reducing the halo phenomenon for electron bombarded devices, such as HPDs, EBAPSs, EBCCDs, MSMVPTs, APDs and resistive anodes. A need also exists for reducing electron backscattering in these devices and, thereby increase gain. The present invention addresses these needs.